While nanotubes, fullerenes, and graphene form the basis of academic and commercial carbon research, other nanoparticulate forms of carbon, such as nanodiamond powder (ND) produced by detonation synthesis (e.g., detonating oxygen-lean explosives in a closed chamber), remain less understood. Currently, large-scale commercial production of nanodiamonds is established in Russia, China, Japan, and several European countries. The rise of interest in nanodiamond is due to its many unique properties, including: superior hardness and Young's modulus, high electrical resistivity, attractive optical characteristics, excellent chemical stability and biocompatibility, all of which it inherits from bulk diamond and delivers on the nanometer scale, in the form of ˜5 nm primary particles with large accessible surfaces bearing a variety of reactive functional groups.
Essentially all existing and future applications of nanodiamonds critically depend on the small particle size (e.g., primary particles having effective diameters of 5-10 nm). They have numerous useful properties and are used in applications ranging from lubricants to drug delivery. However, aggregation of nanodiamond particles is limiting wider use of this important carbon nanomaterial, because most applications require single separated particles. As compared to other nanomaterials, the detonation nanodiamonds have an unusually strong tendency to aggregate, which makes the primary nanodiamond particles very difficult to isolate and keep separated. See, e.g., Kruger, A., et al., Carbon, 2005, 43, 1722-1730. The nature of these nanodiamond aggregates is still not clearly understood. It is hypothesized that the extreme conditions in the detonation wave result in dangling bonds on the surface of nanodiamond particles which, in later stages, when temperature and pressure drop, react and either form strong carbon-carbon covalent interparticle bonds, graphitic shells (which may engulf several nanodiamond primary particles and hold them together in a strongly bonded aggregate) or surface functional groups, which can also cause aggregation via hydrogen bond formation or dipole-dipole and weak Van der Waals interactions between the functional groups on adjacent nanodiamond particles. A. Krueger et al., Carbon, 2005, 43, 1722-1730 have proposed a hierarchical model of the nanodiamond aggregates, subdividing them into agglomerates (20-30 nm), intermediate aggregates (2-3 nm), and core aggregates (100-200 nm). While agglomerates and intermediate aggregates can be disintegrated by mild or powerful sonication, the core aggregates, according to Krueger et al., are very strong and cannot be broken up by any conventional mechanical, ultrasound, or surfactant-assisted techniques. For the core nanodiamond aggregates, a model was proposed in which primary nanodiamond particles are embedded into an amorphous and graphitic carbon matrix holding them together.
With ongoing research and conflicting points of view on the nature of the nanodiamond aggregates, the ultimate goal, however, still remains unchanged: to produce the smallest possible nanodiamond particles and keep them separated. As of now, several techniques are known for the disintegration of nanodiamond aggregates. None but the most aggressive have been successful in providing nanodiamond particles/aggregates having effective diameters less than about 40 nm, and these aggressive methods provide particles with difficult to remove impurities. Additionally, dispersion of the resulting nanoparticulate forms result in large aggregates, in some cases larger than the original aggregated nanodiamond clusters. See, e.g., Xu, K., et al., Solid State Phys., 2004, 46, 633-4 and Xu, X. Y., et al., J. Solid State Chem., 2005, 178, 688-693. Such methods include, for example, gas or liquid phase oxidations (see, e.g., Pichot, V., et al., Diamond and Relat. Mater., 2008, 17, 13-22; Osswald, S., et al., J. Am. Chem. Soc., 2006, 128, 11635-11642; and Xu, K., et al., Solid State Phys., 2004, 46, 633-4), high dynamic pressure pulses (see, e.g., Vul, A. Y., et al., Tech. Phys. Left., 2006, 32, 561-3), or by mechanochemical means, optionally supplemented by high power sonication (see., e.g., Osawa, E., Ed. Springer: New York, 2010; pp 1-33; Ozawa, M., et al., Adv. Mater., 2007, 19, 1201-6; Osawa, E., Diamond and Relat. Mater., 2007, 16, 2018-2022; Xu, X. Y., et al., J. Mater. Sci. Technol., 2005, 21, 109-112; Xu, Y. Y., et al., Diamond and Relat. Mater., 2005, 14, 206-212; Mochalin, V. N., et al., Pharm. Res., 2009, 26, 1365-1370;). In the method of Ozawa, M., et al., Adv. Mater., 2007, 19, 1201-6, called bead-assisted sonic disintegration (BASD), high power sonication of a nanodiamond slurry is combined with milling using zirconia microbeads. BASD resulted in a remarkable decrease in the particle size: nanodiamond particles smaller than 10 nm were produced within 2 hours of sonication. However, zirconia contamination left in the nanodiamonds after either milling or BASD, caused by the attriting nanodiamonds, was very difficult to remove as zirconia is highly resistant to most acids/bases, affecting biomedical and other applications of the nanodiamonds.
Consequently, literature shows that, of the well documented techniques, only zirconia microbeads-assisted wet milling and BASD are currently capable of breaking the core nanodiamond aggregates and producing stable suspensions consisting of primary nanodiamond particles. However, contamination of nanodiamonds with difficult-to-remove zirconia, the high cost of zirconia microbeads, and nanodiamond amorphization (or even graphitization) in the course of milling are major drawbacks of the microbeads-assisted milling.
There still is a need for a simple and cost effective approach to disaggregating aggregated nanodiamond clusters for the mass production of disaggregated nanodiamond powders and colloids.